In certain situations, it is necessary or desirable to know and to be able to reproduce repeatedly and reliably the resolution of an image captured on a medium. In the context of an image captured on film, i.e., as a conventional photograph, the term "resolution" is analogous to the scale of the photograph and indicates how a distance or dimension measured "off of" the photograph corresponds to the distance or dimension in the scene or object that has been photographed. In the context of digital image acquisition using, e.g., a digital "photographic" or "still" camera or a digital video camera, the term "resolution" refers to the number of pixels in the image corresponding to a distance or dimension in the scene or oject being "photographed" or monitored, typically referred to as image pixels per object inch or, more conveniently, dots per inch ("dpi").
One such application where it is necessary to know the resolution of an image (object inch per photograph inch in a film-based photograph or dots per object inch (dpi) in a digital image) relates to photographing fingerprints, e.g., for purposes of crime investigation. When fingerprints are photographed using a conventional, film-based camera, the person taking the picture typically puts an object of known size ("reference object"), e.g., a ruler or a coin, in the field of view with the fingerprint target. Dimensions within the fingerprint, e.g., from one fingerprint landmark to another, are then calculated by measuring the distance as shown in the photograph and scaling that distance (as shown in the photograph) either up or down, the scale factor being equal to the known, actual size of the reference object divided by the size of the reference object as shown in the photograph. Similarly, if a digital camera is used (either still ("photographic") or video), the relevant distance is determined by measuring it in the image in terms of pixels, and then multiplying the number of pixels by an appropriate scale factor expressed in terms of length per pixels, the scale factor being equal to a known dimension of the reference object divided by the number of pixels in the image corresponding to that known dimension.
Calibrating the resolution of the image in this manner can be time-consuming, difficult, and therefore inaccurate. Accordingly, there is a need for an apparatus and method to facilitate recording an image with a known resolution, and doing so repeatedly and reliably.